1. Field of the Invention
This invention relates to uninterruptible power supplies, and, more particularly, to such power supplies having separate charging circuits for charging of the back-up battery supply.
2. Description of the Prior Art
Uninterruptible power supply (UPS) equipment has been produced for many years. Such UPS devices are widely known and used in the art for providing power to loads, such as computers, where AC power loss cannot be tolerated. Thus, in the computing environment, AC power loss results in loss of data and programs running on the computer system at the time of the loss. Moreover, loss of AC power can sometimes result in damage to computer system components. It is recognized that in such environments, and in other applications where power must be maintained, that even momentary loss of power, e.g., less than one cycle of the line current, can result in significant damage, due to loss of data, etc. Prior art UPS devices thus provide large battery packs, or back-up battery supplies, to source power for a relatively short period of time. For a variety of reasons, including size of the UPS, it is important to have an overall UPS design which minimizes the size and complexity of the battery charger. The more complex the battery charger is, the larger and more costly the UPS needs to be.
In prior art designs generally, each UPS has an internal charger designed to recharge the batteries after utility failure has occurred. Thus, in a traditional UPS design, having a rectifier circuit and inverter circuit, the battery is connected to the output of the rectifier, and to the input of the inverter, as illustrated in the prior art design of FIG. 1(a). The rectifier circuit includes a switching circuit, for providing regulated DC from the unregulated utility voltage. In this arrangement, the battery receives the regulated rectified voltage from the rectifier, and supplies power to the inverter in the case of power line loss. The inverter performs the task of changing the power back to AC at power line voltage, for connection to the load. The inverter typically has a PWM (pulse wave modulation) circuit, the switching pattern of which has high frequency components. These components are removed by an LC output circuit, such that the resultant wave form is the fundamental, namely a sine wave. A known problem with this type of prior art circuit is that the rectifier must have the capacity to supply the full system rating plus whatever additional amount is required to recharge the batteries. If the battery charger circuit is complex and draws a high current, this requires use of larger high power components in the rectifier, increasing the cost of the system.
Lower cost prior art systems modify the basic UPS approach by disconnecting the battery from the rectifier during normal operation. As seen in FIG. 1(b), the rectifier network is replaced by two elements, namely an unregulated rectifier network to process the major power component, and a charger circuit to recharge the batteries. By this technique, the charger can be reduced in size, achieving some cost savings. Such charger is, however, traditionally of the phase control type, and contains the basic elements including a voltage transformer for providing isolation from the utility lines and matched voltage to the battery voltage; solid state switches to control the conduction of current from the voltage transformer; an inductor to filter and provide an impedance to limit charge current; and a control circuit that regulates the conduction period of the solid state switches to effect battery charging voltage and charging current control. Thus, the charger circuit duplicates much of the circuitry for providing the main power output. The provision of such a separate charger adds substantially to the cost of the system.
There is thus a need in the art for a UPS having a battery charger circuit which does not duplicate the steps of the main power processing elements, i.e., the power factor correction circuits and inverter circuits which precondition the power drawn from the utility. What is needed is a charger design which utilizes as much of the standard power processing circuitry as possible, and yet provides a low component isolated battery charger for providing high current recharging to the battery supply.